The hypothesis to be tested is that giving topotecan in combination with radiation will kill brainstem glioma cells better than topotecan alone and that children treated this way will have a better outcome. The purposes of this study are 1) to determine the highest dose of topotecan that can safely be given with radiation therapy to children with brainstem glioma; 2) to determine the side effects of this treatment; and 3) to determine whether this drug combination helps control the growth of brainstem glioma.